1. Field of the Invention
The present invention relates to an image display apparatus which has a high contrast and reduces the contrast variation and the effect of unwanted reflections due to the surrounding environment.
2. Description of the Related Art
Conventionally, there have been proposed image display apparatuses of various configurations. As an example of this, there is known an image display apparatus configured as illustrated in a sectional view in FIG. 10A. The image display apparatus 1000 illustrated in FIG. 10A is configured to include a front layer 1001, a luminescent layer 1003 and an excitation source 1004, 1008 for exciting the luminescent layer 1003. The image display apparatus 1000 is configured by arranging a plurality of such configurations. The front layer 1001 is comprised of a medium transparent to visible light such as glass and plastics. The excitation source 1004 is configured, for example, such that an electron emission element and an electrode are arranged on a substrate and the electrode is provided between the front layer 1001 and the luminescent layer 1003. In this configuration, when an electric field is applied to the electron emission element, electrons are released and are supplied to the luminescent layer 1003. Thereby, light is produced by the luminescent layer 1003. Alternatively, as another configuration of the excitation source 1004, 1008, the excitation source 1004, 1008 is configured such that an anode and a cathode are provided on the front surface and the rear surface of the luminescent layer 1003 respectively. The light produced by the luminescent layer 1003 passes through the front layer 1001 and is extracted outside to form display light 1005.
The image display apparatus is required to have a high contrast. In order to improve the contrast of the image display apparatus in a bright place, it is necessary to improve the display luminance as well as to reduce outside light reflected light and reduce the minimum luminance in black display. Here, the outside light reflected light refers to light which is incident on the image display apparatus from outside, is reflected by the image display apparatus, and is emitted to outside. Further, the outside light reflected light can be classified into specularly reflected light and diffusely reflected light. The direction perpendicular to a screen of the image display apparatus is assumed to be the z axis. The specularly reflected light refers to light which is incident on the image display apparatus from a direction of an angle of θ to the z axis and is emitted to a direction of an angle of −θ to the z axis in the incident surface, of the light reflected by the image display apparatus. The diffusely reflected light refers to light other than the specularly reflected light, of the outside light which is incident on the image display apparatus from outside, is reflected by the image display apparatus, and is emitted to outside.
When outside light 1006 is incident on the image display apparatus 1000, strong reflected light occurs on an interface between the front layer 1001 and the excitation source 1004, an interface between the excitation source 1004 and the luminescent layer 1003, and a rear surface of the luminescent layer 1003, forming specularly reflected light 1007. The light reflected from the image display apparatus 1000 forms an image reflecting a surrounding fluorescent lamp and the background. When the light intensity of reflected light is large, a reflected image is recognized on the screen, thus blocking the view of the image display apparatus. Such a phenomenon is referred to as unwanted reflections. In order to increase the display light luminance of the image display apparatus 1000, it is important to reduce loss occurring from when the light is generated by the luminescent layer 1003 until the light is extracted outside. As a cause of this loss, there is a total reflection loss at an interface between the luminescent layer 1003 and the front layer 1001 and an interface between the front layer 1001 and an external region. When light propagates from a high refractive index medium to a low refractive index medium, the light propagating at an angle larger than a critical angle is fully reflected and the light is confined in the high refractive index medium. Such light is not extracted in the low refractive index medium and propagates in the high refractive index medium, consequently causing the loss.
As a method of reducing the total reflection loss and increasing the display light luminance, there has been proposed a method of providing a periodic structure between layers formed of a medium having a different refractive index. For example, Japanese Patent No. 2991183 discloses an organic electroluminescent element configured as illustrated in FIG. 10B. The image display apparatus (organic electroluminescent element) 1100 illustrated in FIG. 10B is configured to include a front layer 1101, a transparent electrode 1102, a luminescent layer 1103, and an electrode layer 1104. A periodic structure 1105 is provided between the front layer 1101 and the luminescent layer 1103. A refractive index distribution structure having a period substantially equal to the wavelength of light is used in the periodic structure 1105. Moreover, it is known that such periodic structure 1105 can increase light propagating at an angle equal to or smaller than the critical angle and can increase light extracted outside by diffracting light generating inside the luminescent layer 1103.